"Some of the herbicides they're using to kill the local plants and vegetation so they can put their plants in there, I've seen some damage from that," said Walter Smith, an environmental protection specialist, "because after they leave, we get the rains and a lot of erosion."

For years, USDA Forest Service officials and other public land managers have seen devastating impacts from chemicals used to illegally grow marijuana on Oregon forest lands.

The Bureau of Land Management said it has seen an increase in illegal grows in recent years.

"Any time you start growing things that are not nautral, you are putting the ecosystem at risk," said Michael Mascari with BLM.

Mascari said the BLM sees two main types of marijuana growers on forest lands: those who plant, walk away and then return when the plants are fully grown; and those who camp out near their crops.

Both types are known to leave pesticides, herbicides, fertilizers, fuels and even human feces behind. Makeshift irrigation systems sometimes divert natural creeks and springs to water the crop.

If you encounter an illegal marijuana grow, you are encouraged to leave the area immediately. Many growers are armed.